1. Field of the Invention
This invention relates to thermal ink jet printing on demand, and more particularly to thermal ink jet printhead configurations and processes for fabricating them.
2. Description of the Prior Art
There are two general configurations for thermal drop-on-demand ink jet printheads. In one configuration, droplets are propelled from nozzles in a direction parallel to the flow of ink in ink channels and parallel to the surface of the bubble generating heating elements of the printhead, such as, for example, the printhead configuration disclosed in U.S. Pat. No. 4,601,777 to Hawkins et al. This configuration is sometimes referred to as "edge or side shooters". The other thermal ink jet configuration propels droplets from nozzles in a direction normal to the surface of the bubble generating heating elements, such as, for example, the printhead disclosed in U.S. Pat. No. 4,568,953 to Aoki et al. This latter configuration is sometimes referred to as a "roofshooter".
In roofshooters, it is often desirable to supply ink to the nozzles via a passageway through the heater plates. This is the obvious choice because the proximity of the paper to the printhead makes any other design approach difficult. In a commerical drop-on-demand thermal ink jet printer sold by the Hewlett-Packard Company known as the Think Jet.RTM., the printhead comprises a heater plate and a fluid distributor plate. The heater plate is a glass substrate having the heating elements and addressing electrodes formed thereon with a hole drilled or isotropically etched, so that the ink can be fed through the heater plate to a shallow reservoir in the fluid distributor plate which is made by electroforming a material such as nickel over a three-dimensional mandrel. The apertures or nozzles in the fluid distributor plate are provided by thick film resist spot patterns formed on the mandrel prior to initiation of the electroform process. When the heater plate and the fluid distributor plate are aligned and bonded together, the contour of the fluid distributor plate forms the shallow reservoir mentioned above and the ink channels to the apertures that serve as droplet emitting nozzles. The ink travels through the drilled or etched hole and across the plane of the heater plate, thus also across the addressing electrodes, to the nozzles. There are two major disadvantages of this configuration. One is that it exposes the electrodes to the ink whenever there are any pinholes in the passivation layer. Secondly, the ink reservoir is quite shallow because it must be formed by the electroform. The shallow reservoir tends to permit the ink to dry out in the nozzles, causing first drop problems.
U.S. Pat. No. 4,601,777 to Hawkins et al discloses a side shooter configuration for a thermal ink jet printhead and several fabricating processes therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A lineal array of parallel grooves also are formed in the second part, so that one end of the grooves communicate with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be made simultaneously by producing a plurality of sets of heating element arrays with their addressing electrodes on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds are produced in a second silicon wafer. In one embodiment, alignment openings are etched in the second silicon wafer at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads.
U.S. Pat. No. 4,568,953 to Aoki et al, discloses a thermal ink jet printhead in which the ink droplets are ejected on demand through nozzles aligned above and parallel to the heating elements, so that the droplet trajectories are normal to the heating elements. Such configuration as indicated above is referred to as a roofshooter. In order to prevent nozzle clogging, the ink is circulated through the printhead in internal passageways having cross-sectional flow areas larger than that of the nozzles. This enables particulate matter larger than the nozzles to pass and be swept away by the circulating ink entering and leaving the printhead through inlet and outlet tubes.
German Patent Application OLS No. 3,402,680, filed Jan. 26, 1984, and published without examination on Aug. 2, 1984, discloses geometrical relationships of nozzle shapes, thicknesses and diameters, as well as nozzle placement relative to the heating elements, for a roofshooter configuration of a thermal ink jet printhead. Some fabrication details are given, including nozzle plates fabricated from stainless steel sheets with the nozzles being etched therein.
U.S. Pat. No. 4,438,191 to Cloutier et al discloses another method of making a roofshooter type thermal ink jet printhead which eliminates need for using an adhesive to construct multiple part assemblies.
U.S. Pat. No. 4,502,060 to Rankin et al, discloses a roofshooter type thermal ink jet printhead having two substantially L-shaped barriers located between an orifice plate and a heating element bearing substrate. The barriers are placed on opposite sides and partially surround the heating elements. The ink feed channels direct the ink to the space interior of the L-shaped barriers from opposite directions to impart an angular momentum to the ink relative to the heating elements during refill after droplet ejection and the bubble collapses.
The present roofshooter type thermal ink jet printheads require that the ink must lie over the passivated electrical leads to the heating elements. This provides a failure mode by shorting through any pinhole in the passivation coating. The ink is fed to the ink reservoir located over the electrical circuitry and heating elements, and this ink reservoir is quite shallow because it is formed by an electroforming process on a shaped mandrel. In addition to the tendency of the shallow reservoir to dry out causing first drop problems, such small capacity ink reservoirs, have high fluid inertia which causes problems in the resupplying of ink to the vicinity of the heating elements when multiple droplets are concurrently fired. These and other problems are solved by the following invention.